I am having a problem with a 1996 Ford Taurus station wagon.  Basically
the 2 problems are #1 I am getting no heat and #2 the engine
temperature gauge is fluctuating.

In my reading people have suggested a number of things which I was
going to try including filling the coolant, replacing the thermostat
and potentially replacing the radiator cap.  Do these fixes sounds
reasonable, where is a good place to buy the thermostat and radiator
cap (local auto store), and are there any other good possibilities that
might be leading to these problems?

Additionally, if you can point me to how to replace the thermostat or
about how much I should be paying for it that would be great.

Thanks in advance!